Hi there Mal and welcome to POSCON!
I am not that certain I got your question. You should only tick that box if your callsign is your aircraft registration (e.g. OECFL or HV123B). If you have a specific callsign that contains the ICAO code of an airline, scheduled or unscheduled, or anything of the like you should enter that one there (e.g. EWG2WE or AUA1L) and add the registration in any case in the other fields box with REG/OEIEU or REG/OELBA etc.